Pressure sensor means of the type indicated above are described e.g. in the Swedish patent application 8700556-7 which is available to the public and has been filed by the applicant.
For pressure gauges of the kind mentioned above various problems are raised in relation to the attachment of said pressure sensor means to the interior surface of the surrounding hollow volume. Generally the desires for and demands on a pressure gauge having a high precision, intended both for measuring low pressures and pressures originating from corrosive gases, may be summarized in the following way.
1. The whole of said pressure sensor means should be freely hanging or standing in the measuring volume inside the hollow volume in such a way that the pressure being measured, besides acting on said diaphragm, also acts on the whole of said pressure sensor means. No forces should influence the mechanical construction, particularly the support plate, of the pressure sensor means, which is displaced due to the relation of the measured pressure to the reference pressure inside said pressure sensor means.
A mounting of this kind will among other things result in the fact that the values provided by said pressure sensor means are given an increased repeatability when the pressure sensor means is subjected to varying pressures.
2. The freely hanging or standing pressure means should be mounted in such a way that the means attaching the pressure sensor means to the inner surface of the surrounding hollow volume has a small sectional area compared to the large, substantially flat surfaces of the pressure sensor means.
Such an attachment of the pressure sensor means naturally has a relation to the desire mentioned under point 1. that no forces or as small forces as possible, besides the pressure to be measured, should influence the pressure sensor means. In addition a reduced temperature convection may in this way be obtained between the walls of the surrounding hollow volume and the pressure sensor means.
3. The attachment means by means of which the pressure sensor means is attached inside the hollow volume should be mechanically connected partly to the pressure sensor means, partly to the interior wall of the surrounding hollow volume in such a way that the connecting joints do not comprise any type of soldering or elastomer. Only weld joints and glass joints should be allowed to be used.
This requirement has a direct connection to the object of being able to use the pressure gauge for measurement of both very low pressures and for pressures generated by corrosive media.
4. The electric conductors connecting the electrodes of said pressure sensor means to the exterior of the surrounding hollow volume should not be subjected to the gases in the measuring volume. If the electric conductors were to pass through said measurement volume this would influence the capacitive measurement or totally disable this.
First it will be observed that the desire under point 3. for certain cases possibly could be obtained by means of a reconstruction of constructions known in the art, e.g. by a choice of material allowing mechanical connections such as welds or glass joints.
However, the requirements or desires under the points 1. and 2. may be satisfied in different ways, for instance as in illustrated in the Japanese patent applications 56-141531 and 58-26237 and the U.S. Pat. No. 4,358,814. In these constructions known in the art the mechanical means connecting the support plate of said pressure sensor means to the wall of the surrounding hollow volume is located centrally and in addition has a small section in relation to the area of the large surfaces of said pressure sensor means.
In the cited U.S. patent publication this attachment means is realized in the shape of a tube. However, this pressure gauge known in the art is not totally comparable to the type of pressure gauge to which the present application is related, since said support plate in the previously known construction has not a substantially flat shape.
The requirement indicated under point 4. is satisfied according to the invention by the features set out in the characterizing part of the claim 1.
Thus, the pressure sensor means is attached to and only connected to the wall of the surrounding hollow volume by means of at least one tube, the section of which is small in relation to the area of the large surface of the support plate and the diaphragm of said pressure sensor means. This can be achieved by the fact that the connecting leads to the sensing electrodes of said pressure sensor means pass inside this tube or these tubes.
In this context reference can be made to the U.S. Pat. Nos. 3,619,742 and 4,168,517. In these patent publications pressure gauges are disclosed having pressure sensor means, the flexible diaphragm and a rigid support plate of which are influenced by different pressures, e.g. by the measurement pressure and substantially the atmospheric pressure respectively. However, in these kinds of pressure gauges it may also be advantageous to make the electric connection or the electric connections pass through a tube, in this case generally made of metal and providing an electrical shielding of the measurement wire. In these cases, however, see particularly the U.S. Pat. No. 3,619,742, said tube has no carrying or supporting effect.